1. Field of the Invention
The present invention relates to a resonant tunneling structure.
2. Description of the Related Art
In recent years, research and development for applying a terahertz-wave (an electromagnetic wave having a frequency of 30 GHz to 30 THz in the present specification) to industrial fields including communications, securities, and medical cares have been actively conducted. The terahertz-wave has substance penetrability and rectilinear propagation characteristics, and hence internal information on an object can be obtained at high resolution based on a reflection signal or a transmission signal from the object. Therefore, various nondestructive or noninvasive inspection technologies have been developed as described below.
(1) Technology using safe fluoroscopic imaging of an object in place of X-ray imaging
(2) Spectral technology for obtaining absorption spectrum or complex dielectric constant of a substance to examine binding state of molecules
(3) Technology of evaluating carrier concentration or mobility of a superconducting material
(4) Technology of analyzing a biomolecule (DNA or protein)
One of the essential development elements to put the technologies described above to practical use is a development of a terahertz light source. Up to now, generating methods using a laser device such as a femtosecond-laser-excitation photoconductive device or a terahertz-wave parametric oscillator with a nonlinear optical crystal have been developed. Generating methods using a small electron vacuum tube such as a backward wave oscillator (BWO) or a gyrotron or using a large electron beam accelerator such as a free-electron laser have been also developed. Because of variable frequency and a large output power, the methods are highly effective in special uses including the identification of fingerprint spectrums of various materials. However, the methods require a large excitation light source or vacuum tube, and hence there is a limit on reductions in size and power consumption of the device.
Therefore, for example, a structure using a quantum cascade laser or a resonant tunneling diode (hereinafter, referred to as RTD) has been studied for current injection type oscillation devices which operate in a terahertz wave region. The oscillation devices are used to realize electromagnetic wave oscillation based on intersubband transition of electrons in a semiconductor quantum well structure.
An RTD type oscillation device as described in Japanese Patent Application Laid-Open No. 2007-124250 is expected to operate in the vicinity of 1 THz at room temperature.
According to Japanese Patent Application Laid-Open No. 2007-124250, a device using a double-barrier RTD as an active layer is provided. In voltage-current (V-I) characteristics of the device as illustrated in FIG. 10, a negative differential resistance 1001 is observed. The double-barrier RTD includes an InGaAs quantum well layer and an AlAs tunnel barrier layer which are epitaxially grown on an InP substrate. When the double-barrier RTD is incorporated as the active layer into a resonator having an antenna structure two-dimensionally formed on the surface of the substrate, an electromagnetic wave 1002 can be oscillated in the vicinity of the region of the negative differential resistance 1001 as illustrated in FIG. 10.
When the device using the double-barrier RTD as the active layer is provided, an oscillation frequency and an oscillation output are limited by a gain resulting from the negative differential resistance 1001 or by an injectable current density.
In view of the oscillation output, it is desirable to increase a peak-to-valley ratio which is a ratio between a peak current value in the voltage-current (V-I) characteristics and a gradient in the negative differential resistance region.
According to Japanese Patent Publication No. H06-030399, in order to increase the peak-to-valley ratio of the RTD, a triple-barrier RTD is proposed which includes a quantum well layer of In0.07Ga0.93As, a quantum well layer of GaAs, and a tunnel barrier layer of AlAs, which are epitaxially grown on a GaAs substrate. The two quantum well layers have two mutually different compositions. The triple-barrier RTD includes the quantum well layers described above, and hence a peak current can be generated by a resonant tunneling phenomenon occurring when a voltage is applied. Therefore, the increase in peak-to-valley ratio is realized.
With respect to the frequency of the RTD type oscillation device, it is desired that a single device has multiple (for example, two) frequencies. However, when the structure disclosed in Japanese Patent Application Laid-Open No. 2007-124250 or Japanese Patent Publication No. H06-030399 is used, only a single frequency oscillation can be realized.